SAML: Allow specifying the IdP entityid to use. (#8630)

If the SAML metadata includes multiple IdPs it is necessary to
specify which IdP to redirect users to for authentication.

changelog.d/8630.feature

+Allow specification of the SAML IdP if the metadata returns multiple IdPs.

docs/sample_config.yaml

@@ -1674,6 +1674,14 @@ saml2_config:
   #  - attribute: department
   #    value: "sales"
 
+  # If the metadata XML contains multiple IdP entities then the `idp_entityid`
+  # option must be set to the entity to redirect users to.
+  #
+  # Most deployments only have a single IdP entity and so should omit this
+  # option.
+  #
+  #idp_entityid: 'https://our_idp/entityid'
+
 
 # Enable OpenID Connect (OIDC) / OAuth 2.0 for registration and login.
 #

synapse/config/saml2_config.py

@@ -90,6 +90,8 @@ class SAML2Config(Config):
             "grandfathered_mxid_source_attribute", "uid"
         )
 
+        self.saml2_idp_entityid = saml2_config.get("idp_entityid", None)
+
         # user_mapping_provider may be None if the key is present but has no value
         ump_dict = saml2_config.get("user_mapping_provider") or {}
 
@@ -383,6 +385,14 @@ class SAML2Config(Config):
           #    value: "staff"
           #  - attribute: department
           #    value: "sales"
+
+          # If the metadata XML contains multiple IdP entities then the `idp_entityid`
+          # option must be set to the entity to redirect users to.
+          #
+          # Most deployments only have a single IdP entity and so should omit this
+          # option.
+          #
+          #idp_entityid: 'https://our_idp/entityid'
         """ % {
             "config_dir_path": config_dir_path
         }

synapse/handlers/saml_handler.py

@@ -58,6 +58,7 @@ class SamlHandler(BaseHandler):
     def __init__(self, hs: "synapse.server.HomeServer"):
         super().__init__(hs)
         self._saml_client = Saml2Client(hs.config.saml2_sp_config)
+        self._saml_idp_entityid = hs.config.saml2_idp_entityid
         self._auth_handler = hs.get_auth_handler()
         self._registration_handler = hs.get_registration_handler()
 
@@ -100,7 +101,7 @@ class SamlHandler(BaseHandler):
             URL to redirect to
         """
         reqid, info = self._saml_client.prepare_for_authenticate(
-            relay_state=client_redirect_url
+            entityid=self._saml_idp_entityid, relay_state=client_redirect_url
         )
 
         # Since SAML sessions timeout it is useful to log when they were created.